The Hubble Space Telescope uses mirrors to collect science
data. Hubble's main mirror is about eight feet in diameter.
Light enters the telescope and strikes the main mirror, which
reflects the light forward to a smaller (12-inch) secondary
mirror. This small mirror reflects the light again, sending
it through a two-foot hole in the center of the large mirror
to the science instruments. These powerful instruments analyze
the incoming light stream and translate it into information
and images for scientists back on Earth.

After Hubble's launch in 1990, NASA discovered a flaw in
the large, main mirror. The flaw was tiny  about 1/50th
the thickness of a piece of paper  but significant enough
to distort Hubble's vision. During the First Servicing Mission,
astronauts added corrective optics to compensate for the flaw.
The optics acted like eyeglasses to correct Hubble's vision.
(Longer Caption)